A critical problem in understanding the natural history of breast cancer and in developing more effective methods for determining prognosis and treatment is the paucity of markers with which to classify these tumors. Considerable progress has been made, for example, in the management of childhood leukemia as a consequence of the ability to subdivide leukemias by the cell of origin for each individual cancer. We here propose to develop specific methods to classify human breast cancers based on the identification of their cellular origins. We will determine whether such a sub-classification of breast cancers is related to the specific characteristics of the tumor and whether the cellular origin of an individual breast tumor is related to its biological behavior. To accomplish these goals, we will develop and characterized monoclonal antibidies specific for each of the 3 types of epithelial cells of the normal human mammary gland. Antibody-producing cells, raised in mice immunized with organoids of normal breast tissue derived by collagenase digestion of human mammoplasty reduction specimens, will be fused with a mouse myeloma cell line. Resultant hybridomas will be cloned twice and their monoclonal antibodies characterized by reaction with specific cell types in sections of normal and lactating human breast tissue. The panel of monoclorals will be tested by immunoperoxidase staining to determine the cellular origins of a series of 900 well-characterized human breast cancers, followed clinically for up to 7 years and already collected for the study. We will determine correlations between cellular origins and specific characteristics and biological behavior of these tumors, i.e., histopathological type, estrogen receptor content, cell surface characteristics, invasiveness, site of metastasis, time to recurrence and response to therapy. We will use these antibodies to characterize outgrowth colonies of normal and neoplastic breast tissue in vitro, enhancing the value of these model systems for studies of breast physiology, carcinogenesis and therapeutic strategy. It is the goal of this study to provide markers for the cellular origins of breast cancer which correlate with the biological behavior of indivividual tumors and give insight into the pathophysiology and natural history of human breast cancers.